PushButton library converts & simulates the smartphone as a real Hardware push button so that you can control physical objects surrounding you.
Adding #define CUSTOM_SETTINGS along with #define INCLUDE_PUSH_BUTTON_SHIELD is a key that will let you unlock the Push Button shield only amongst all shields and prevent 1Sheeld library from eating your memory specially for UNO board.
&userFunctionName: takes the physical address of the userFunction in the memory and jump to it once called to execute it’s functionality.
/* myFunction exists under the loop function. */
/* This function will be invoked each time a the button is pressed. */
void myFunction (byte isButtonPressed)
/*Turn on a LED if button pressed. */